It has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, extrusion of aluminum--lithium alloys using conventional flat faced extrusion dies to produce thick and/or low aspect ratio aluminum--lithium extrusions results in materials which sometimes display low elongations, poor fracture toughness and high anisotropy of mechanical properties. These characteristics have been associated with cracking problems in the extruded product during certain fabrication procedures, especially during machining operations which are performed on some extrusions. There is also concern regarding the load transfer capability of thick and/or low aspect ratio aluminum--lithium extrusions. This is because such extrusions have a small spread between ultimate tensile strength (UTS) and tensile yield strength (TYS). This problem is also sometimes present in portions of the extruded shape which are formed by generally axisymmetrical metal flow, even if the portion has a high aspect ratio.
As used in the art, the term aspect ratio means the ratio of the width to thickness in cross section of an extrusion or a portion of an extrusion. A low aspect ratio is in the range of approximately 1-4:1. A 1:1 ratio means that the extrusion or the portion of the extrusion has approximately a round or square cross section. A ratio of 4:1 means that the width of the extrusion would be approximately 4 times the thickness of the extrusion in cross section.
The influence of the extrusion process parameters on mechanical properties of different aluminum--lithium extrusions was investigated by Tempus et al as published in Aluminum Lithium, Vol. 4, (1987). The strength of the extrusion was shown to be influenced more by the extrusion aspect ratio, (width/thickness) which determines texture, than by extrusion temperature and extrusion ratio. With increasing extrusion aspect ratio, the strength declines considerably. Tempus et al revealed that a small difference between UTS and TYS in aluminum--lithium extrusions is associated with a fiber texture. If an extrusion does not undergo recrystallization during solution heat treatment, the region of the extrusion where the flow resembles axisymmetric deformation exhibits a fiber texture. In contrast, when deformation conditions are more like plane strain, the resulting extrusion or section of an extrusion contains a more equiaxed type texture. The later is typical of extrusions with a high aspect ratio which have an acceptable difference between UTS and TYS and which will machine well with little or no cracking.
A paper entitled Texture and Properties of 2090, 8090 and 7050 Extruded Products, by D. K. Denzer, P. A. Hollingshead, J. Liu, K. P. Armanie and R. J. Rioja, which was presented at the Sixth Annual International Aluminum--Lithium Conference, Garmisch-Partenkirchen, Germany, Oct. 7-11, 1991, contains a further discussion of the correlation between extrusion aspect ratio and tensile strength in aluminum--lithium extrusions.
U.S. Pat. No. 5,151,136 discloses a method for producing low aspect ratio aluminum--lithium extrusions with improved properties in which the low aspect ratio section of the extrusion is reduced by at least 4:1 (extrusion ratio) during the extrusion reduction.
A method is desired for producing aluminum--lithium alloy extrusions having portions thereof with a low aspect ratio with improved elongation, improved fracture toughness and other mechanical properties.